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Date: Fri, 21 Jul 2006 13:05:13 -0700
From: glen herrmannsfeldt <gah@ugcs.caltech.edu>
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Subject: Re: ONES COMPLEMENT
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Date: 2006-07-21T13:05:13-07:00
List-Id: <comp.lang.ada>

Shmuel (Seymour J.) Metz wrote:
(snip)

> Robin wrote "A properly designed 1s complement machine would not
> generate -0 in normal arithmetic operations."

> I responded "0+(-0)?".

> You responded "A subtractive adder."

> I'm trying to say that there's nothing about 0+(-0) requiring a
> subtractive adder.

Well, even more, the way I understand the logic of some machines,
as long as no arguments are negative zero they won't generate a
negative zero.  0+(-0) doesn't satisfy that case.

For those who care, consider that ones complement arithmetic
will normally generate -0 instead of +0.  Adding x and -x generates
all ones with no end around carry.  If you add the ones complements
of the two operands you get the complement of the result, and still
negative zero for zero sum.  The complement of that will be the correct
sum, and won't be negative zero unless both of the arguments are -0.

In addition, note that ECL supplies a signal and its complement without
any extra logic needed.

In any case, a high level language has to get it right even
if it takes extra instructions.

-- glen